Question ii): Causes of early 20th century warming
An analysis of the connection between temperature and circulation based on the newly available 20th century reanalysis data can disentangle the contribution to early 20th century warming from circulation and from greenhouse gas, volcanic, and solar forcing. This analysis is based on integrating information from multiple data sources and makes full use of the spatial and temporal pattern of warming.
The early 20th century warming (1900-1950) was caused by a combination of a hiatus in volcanic eruptions, greenhouse warming (already modified by aerosols), and an unusual mode of climate variability that led to an anomalously cold starting point (which has not been recognized so far) and a very warm end point. This magnitude of this variability is reproduced in some climate models. This is shown in a quite nicely cited review paper (Hegerl et al., WIRES 2018) drawing on several strands of work in TITAN.